There are many methods of manufacture of thiophene, the starting materials being very varied:
1-butene or butadiene and CS.sub.2 (French Patent No. 2,139,177), PA1 butane or 1-butene, H.sub.2 S and sulphur (French Patent No. 2,242,391), PA1 butanediol and vaporized sulphur (Belgium Patent No. 671,591), PA1 n-butanol or 1,4-butanediol and CS: (French Patent No. 2,139,177), PA1 crotonaldehyde and H.sub.2 S (French Patent No. 2,375,227), PA1 furan and H.sub.2 S (Belgium Patent No. 623,801). PA1 precipitation by addition of sodium sulphide to a methanolic solution of metal chloride(s), then extraction of the sodium chloride and finally treatment of the solid at a temperature of between 300.degree. and 600.degree. C., preferably at approximately 400.degree. C. in the presence of a mixture of hydrogen and H.sub.2 S (2 to 50% by volume, preferably approximately 15%). PA1 impregnation of the support with one or more metal salts, for example chloride(s), followed by direct sulphurization with a mixture of hydrogen and H.sub.2 S (2 to 50% by volume, preferably approximately 15%) or with H.sub.2 S diluted in an inert gas (for example nitrogen) at a temperature of between 300.degree. and 700.degree. C., preferably between 400.degree. and 600.degree. C. and more particularly at approximately 500.degree. C.
This last process is industrial but it requires a cheap supply of furan, which is not the case in most of the world. The other processes present problems of selectivity as a result of the formation of many undesirable products. In addition, they are catalytic processes in which rapid coking of the catalysts is observed, and this requires frequent regeneration.
A starting material which is particularly attractive for the manufacture of thiophene is tetrahydrothiophene (referred to by the initials THT hereinafter), whose industrial synthesis is carried out by starting with tetrahydrofuran, a low-cost raw material. A plant for the manufacture of thiophene can be advantageously placed following an industrial synthesis of THT from tetrahydrofuran, without requiring an excessively great capital cost. In addition, THT can be easily separated from thiophene; it is therefore unnecessary to work at total conversion and this allows the output of the industrial unit to be adapted to the demand.
The dehydrogenation of THT has formed the subject of several investigations. It is known, in fact, to employ oxides of aluminum and of cobalt or of aluminum, cobalt and molybdenum as a catalyst for this reaction (R. D. Obolentsev, Khim. Seroorgan. Soedin. Soderzhasch. v. Nelft. 1 Nefteprod. Akad. Nauk USSR, Bashkirsk. Filial 4, 245-255, 196I and Bashkirsk. Filial 7, 148-155, 1964), but good yields are obtained in the first moments of the reaction and are not permanent. The activity of the catalyst drops very quickly because of coking, which blocks the dehydrogenation reaction.
Oxidative dehydrogenation of THT with SO.sub.2 in the presence of a catalyst (metal oxide, activated alumina or active carbon) is derived in French Patent No. 2,071,189. It is indicated therein that the best results are obtained above all between 500 and 600.degree. C. and that, when the temperature is too low, deactivation of the catalyst is considerable, chiefly as a result of the sulphur condensation, and that, moreover, decoking is incomplete.
In a paper titled "Study of the mechanism of thiophene formation in the presence of sulfide catalysts" (Geterog. Katal. 1979, 4th, pt. 2, 237-42; Chemical Abstracts, vol. 93, 131779d), T. S. SUKHAREVA, et al. teach that the most active catalysts are the sulphides of rhenium, molybdenum, iron, and platinum."